For power plants, which operate together in an interconnected network system, there are certain requirements as regards the reserve power to be held in readiness. Since electric energy is not storable to a significant extent, a balance must always be produced in an interconnected network system between the power fed in by the power plants and the power removed by consumers. In normal operation, this power balance is achieved at a desired frequency, which is 50 Hz in Europe, whereby the power plant generators, which are coupled to the mains network in a fixed frequency relationship, rotate at constant speed. Any disturbance to the power balance results in a change in speed and thus in frequency. The maintenance of the power balance is achieved by the power plants involved by the holding of reserve power available. In order to be able to react to disturbances or nearly step shaped load changes, reserve power must be held in readiness by the producer. Thus, for instance, in accordance with the current regulations in Germany or Europe, the effective electric power of a participating power plant unit must be increased by at least two percent within 30 s (see Verband der Netzbetreiber VDN e. V., at the VDEW, ‘Transmission Code 2007—Netz—and Systemregeln der deutschen Übertragungsnetzbetreiber’, version 1.1, Berlin, August 2007, Chapter 3.3.7.1, page 27/90 and UCTE Operation Handbook, Policy P1, Brussels, V2.2, 20.07.2004).
In a ‘fuel operated power plant’, meaning here a power plant burning fossil fuels or a nuclear power plant, such a rapid change in the power produced by the power plant cannot be achieved by changing the fuel flow (change in the amount of fossil fuel supplied or change in the position of the control and absorber rods in a nuclear power plant). For this reason, the turbine inlet valves are usually not completely opened in such power plants (so that the power output would depend only on the outlet pressure of the steam generator) but are instead throttled to make a spinning reserve power output available by opening that valves. Another known possibility for making a rapid reserve usable within seconds available resides in briefly and temporarily switching off high pressure or low pressure preheaters on the steam and/or water side in order to reclaim the energy branched off from the turbines for these elements (i.e. to leave it in the turbines).
Of disadvantage with these known measures is the reduction in the efficiency of the plant or an uneconomic provision of reserve power, which is associated, for instance, with the persistent throttling of the turbine inlet valves. It is therefore the object of the invention to avoid these disadvantages.